In recent years, an amplification circuit called LINC (linear amplification with nonlinear components) has been attracting attention to improve power efficiency of an amplifier equipped in a wireless communication device, for example. LINC is an amplification circuit that uses an outphasing scheme, and that divides an input signal into signals having different phases with a constant amplitude, and amplifies each of the divided signals with respective amplifiers.
Specifically, for example, as indicated on an IQ plane in FIG. 6, an input signal S is divided into a signal S1 and a signal S2 with an amplitude of Vm. The signal S1 and the signal S2 are amplified by respective amplifiers, and output signals of the respective amplifiers are combined. At this time, if the power of the input signal S varies, although the phase difference between the signal S1 and the signal S2 varies, the amplitude is constant at Vm. Therefore, each of the amplifiers amplifies signals at a constant power level, and occurrence of intermodulation distortion can be suppressed. Accordingly, it is possible to activate an amplifier in a nonlinear region in which linearity of input and output cannot be maintained, and the power efficiency can be improved by keeping the input power level for an amplifier comparatively high.
In LINC, as indicated in FIG. 6, the input signal S is divided into the signal S1 and the signal S2 having different phases. At this time the amplitude of the input signal S is reflected in the phases of the signal S1 and the signal S2. That is, when the amplitude of the input signal S increases, the phase difference between the signal S1 and the signal S2 decreases. On the other hand, when the amplitude of the input signal S decreases, the phase difference between the signal S1 and the signal S2 increases. In FIG. 6, as the amplitude of the input signal S decreases, the angle between the signal S1 and the signal S2 increases, and an unnecessary outphasing component e increases. Therefore, when LINC is used, the power efficiency for an input signal having small amplitude is relatively low.
Therefore, dividing an input signal equally to signals having a phase identical to that of the input signal and performing linear amplification of each of the divided signals by amplifiers when the amplitude of the input signal is small, while applying LINC when the amplitude of an input signal is large have been considered.    Patent document 1: U.S. Pat. No. 7,729,445    Patent document 2: Japanese Laid-open Patent Publication No. 2008-167289    Non-patent document 1: Hongtak Lee, and three others, “A CMOS Power Amplifier for Multi-mode LINC Architecture”, Radio and Wireless Symposium (RWS), 2010, pp. 41-44
However, in the above technique, there is a problem that the power efficiency cannot be sufficiently improved when the amplitude of an input signal is small. That is, as described above, in LINC, as the amplitude of an input signal becomes smaller, the phase difference between two signals that are obtained by dividing the input signal becomes larger, and an unnecessary outphasing component increases. To cope with this outphasing component, a back-off of the amplifier increases, and the power efficiency is deteriorated.
Moreover, in the technique in which an input signal is equally divided to perform linear amplification when the amplitude of the input signal is small, the amplitudes of the equally divided signals are not constant, and therefore, intermodulation distortion can occur in an amplifier, or out-of-band emission can occur in output of the amplifier. Further, to suppress the intermodulation distortion or the out-of-band emission, the back-off of the amplifier increases, and the power efficiency is deteriorated.